Tunable resonant inductive coil systems for wireless power transfer and near field communications

ABSTRACT

A tunable resonant inductive coil system includes an electrical circuit having an alternating current (AC) voltage source, a barium strontium titanate (BST) variable capacitor coupled in series with a first terminal of the AC voltage source, a coil coupled in series with the BST variable capacitor, and a return line coupling the coil with a second terminal of the AC voltage source and/or a ground. The electrical circuit forms an LC circuit (resonant circuit). The electrical circuit adjusts between two configurations. In the first configuration the resonant circuit has a first resonant frequency configured for wireless power transfer and in the second configuration it has a second resonant frequency configured for near field communication (NFC). An entire length of the coil is used for both resonant frequencies. Adjusting between the first and second configurations includes varying a capacitance of the BST variable capacitor in response to receiving a control signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This document claims the benefit of the filing date of U.S. ProvisionalPat. App. 62/134,445 titled “Tuning Inductive Coils for WirelessCharging” to Gareth Pryce Weale, filed Mar. 17, 2015, the disclosure ofwhich is incorporated entirely herein by reference.

BACKGROUND

1. Technical Field

Aspects of this document relate generally to systems and methods forwireless power transfer, such as wireless battery charging. Aspects ofthis document relate generally to systems and methods for near fieldcommunication (NFC).

2. Background

Wireless power transfer uses coupled coils to transfer power from onecoil to another. Wireless power transfer using inductive coupling mayinclude changing a current through a first coil to induce a voltageacross a second coil. Wireless power transfer using resonant inductivecoupling may include the use of a first coil included in a firstresonant circuit and a second coil included in a second resonantcircuit, the two resonant circuits tuned to resonate at the samefrequency. Near field communication (NFC) is a communication protocolenabling electronic devices to transfer data when brought within closeproximity.

SUMMARY

Implementations of tunable resonant inductive coil systems may include:an electrical circuit including an alternating current (AC) voltagesource, a barium strontium titanate (BST) variable capacitor coupled inseries with a first terminal of the AC voltage source, a coil coupled inseries with the BST variable capacitor, and a return line coupling thecoil with one of a second terminal of the AC voltage source and aground; wherein the electrical circuit forms an LC circuit (resonantcircuit); wherein the electrical circuit is configured to, in responseto receiving one or more electrical signals, adjust between a firstconfiguration and a second configuration, wherein in the firstconfiguration the resonant circuit has a first resonant frequencyconfigured to be used for wireless power transfer and in the secondconfiguration the resonant circuit has a second resonant frequencyconfigured to be used for near field communication (NFC), wherein anentire length of the coil is used for both the first resonant frequencyand the second resonant frequency, and; wherein adjusting the electricalcircuit between the first configuration and the second configurationincludes varying a capacitance of the BST variable capacitor in responseto receiving a control signal at the BST variable capacitor.

Implementations of tunable resonant inductive coil systems may includeone, all, or any of the following:

The coil may include an antenna.

Adjusting the electrical circuit between the first configuration and thesecond configuration may include one of opening and closing a switch ofthe electrical circuit to one of electrically couple a fixed capacitorwith the resonant circuit and electrically isolate the fixed capacitorfrom the resonant circuit.

The system may include a second BST variable capacitor coupled in serieswith the coil on the return line.

The system may further include a fixed capacitor coupled in parallelwith the second BST variable capacitor and coupled in series between thecoil and one of the second terminal of the AC voltage source and theground.

The system may further include a fixed capacitor coupled in seriesbetween the first terminal of the AC voltage source and the coil.

The system may further include a fixed capacitor coupled in seriesbetween the coil and one of the second terminal of the AC voltage sourceand the ground.

The system may further include a fixed capacitor coupled in parallelwith the BST variable capacitor and in series between the first terminalof the AC voltage source and the coil.

The first resonant frequency may be between 5.800 MHz and 7.525 MHz andthe second resonant frequency may be between 9.625 MHz and 14.850 MHz.

Implementations of tunable resonant inductive coil systems may include:an electrical circuit consisting of an alternating current (AC) voltagesource; one or more barium strontium titanate (BST) variable capacitors,at least one of the one or more BST variable capacitors coupled inseries with a first terminal of the AC voltage source; a coil coupled inseries with the first terminal of the AC voltage source; a return linecoupling the coil with one of a second terminal of the AC voltage sourceand a ground; a switch coupled with the AC voltage source; a directcurrent (DC) voltage source controlling the switch, and; one or moreadditional passive electrical components; wherein the electrical circuitforms an LC circuit (resonant circuit); wherein the electrical circuitis configured to, in response to receiving one or more electricalsignals, adjust between a first configuration and a second configurationby one of opening and closing the switch and by correspondingly varyinga capacitance of the one or more BST variable capacitors, wherein in thefirst configuration the resonant circuit has a first resonant frequencyconfigured to be used for wireless power transfer and in the secondconfiguration the resonant circuit has a second resonant frequencyconfigured to be used for near field communication (NFC), and; whereinan entire length of the coil is used for both the first resonantfrequency and the second resonant frequency.

Implementations of methods of use of tunable resonant inductive coilsystems may include: providing an electrical circuit, the electricalcircuit including an alternating current (AC) voltage source having afirst terminal, a variable capacitor electrically coupled in series withthe first terminal of the AC voltage source, a coil electrically coupledin series with the variable capacitor, and a return line electricallycoupling the coil with one of a second terminal of the AC voltage sourceand a ground, the electrical circuit including an LC circuit (resonantcircuit); in response to the electrical circuit receiving a controlsignal at the variable capacitor, adjusting a capacitance of thevariable capacitor to adjust a resonant frequency of the resonantcircuit between a first resonant frequency configured to be used forwireless power transfer and a second resonant frequency configured to beused for near field communication (NFC), and; using an entire length ofthe coil for both the first resonant frequency and the second resonantfrequency.

Implementations of methods of use of tunable resonant inductive coilsystems may include one, all, or any of the following:

Adjusting the resonant frequency between the first resonant frequencyand the second resonant frequency may include adjusting the resonantfrequency from a lower value to a higher value and/or adjusting theresonant frequency from the higher value to the lower value, wherein thelower value is between 5.800 MHz and 7.525 MHz and wherein the highervalue is between 9.625 MHz and 14.850 MHz.

The variable capacitor may include a barium strontium titanate (BST)variable capacitor.

The coil may include an antenna.

The variable capacitor may not be a switched capacitor, a varactordiode, or a trimmer capacitor.

Adjusting the resonant frequency between the first resonant frequencyand the second resonant frequency may include either opening or closinga switch of the electrical circuit.

Either opening or closing the switch may electrically couple a fixedcapacitor in parallel with the coil.

The method may further include tuning the capacitance of the variablecapacitor to adjust the first resonant frequency and/or the secondresonant frequency in response to the resonant circuit being out ofresonance with a second resonant circuit of a computing device.

Tuning the capacitance of the variable capacitor may include adjustingthe first resonant frequency from a first value to a second value and/oradjusting the second resonant frequency from a third value to a fourthvalue, wherein the first value and the second value are between 5.800MHz and 7.525 MHz and wherein the third value and the fourth value arebetween 9.625 MHz and 14.850 MHz.

The electrical circuit may include a near field communication (NFC) chipcoupled with the resonant circuit and adjusting the resonant frequencyto the second resonant frequency may protect the NFC chip from wirelesspower transfer signals from a power source by detuning the resonantcircuit from the wireless power transfer signals.

The foregoing and other aspects, features, and advantages will beapparent to those artisans of ordinary skill in the art from theDESCRIPTION and DRAWINGS, and from the CLAIMS.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations will hereinafter be described in conjunction with theappended drawings, where like designations denote like elements, and:

FIG. 1 is a graph plotting current versus frequency for a plurality ofcoils;

FIG. 2 is a diagram of a resonant inductive coil system;

FIG. 3 is a diagram of an implementation of a tunable resonant inductivecoil system;

FIG. 4 is a diagram of an implementation of a tunable resonant inductivecoil system;

FIG. 5 is a diagram of an implementation of a tunable resonant inductivecoil system;

FIG. 6 is a diagram of an implementation of a tunable resonant inductivecoil system;

FIG. 7 is a diagram of an implementation of a tunable resonant inductivecoil system;

FIG. 8 is a diagram of an implementation of a tunable resonant inductivecoil system;

FIG. 9 is a top view of an implementation of a coil used with a tunableresonant inductive coil system;

FIG. 10 is a top perspective view of a plurality of resonant coils usedin a method of use of a tunable resonant inductive coil system;

FIG. 11 is a graph plotting resonant coupling versus frequency for theplurality of resonant coils of FIG. 10 at a separation distance of 3.0cm;

FIG. 12 is a diagram of an implementation of a tunable resonantinductive coil system;

FIG. 13 is a graph plotting voltage versus frequency for the tunableresonant inductive coil system (system) of FIG. 12 when a resonantcircuit of the system is adjusted to a resonant frequency of 6.250 MHzand when the resonant circuit is adjusted to a resonant frequency of13.560 MHz;

FIG. 14 is a graph plotting voltage versus frequency for the tunableresonant inductive coil system (system) of FIG. 12 when the system is ina configuration for wireless power transfer (WPT) and showing a resonantfrequency range from a minimum resonant frequency (5.800 MHz) to amaximum resonant frequency (6.525 MHz) by varying a capacitance of avariable capacitor of the system;

FIG. 15 is a graph plotting voltage versus frequency for the tunableresonant inductive coil system (system) of FIG. 12 when the system is ina configuration for near field communication (NFC) and showing aresonant frequency range from a minimum resonant frequency (9.625 MHz)to a maximum resonant frequency (14.850 MHz) by varying a capacitance ofa variable capacitor of the system;

FIG. 16 is a graph plotting voltage versus time for a tunable resonantinductive coil system of FIG. 12 using a voltage booster, and;

FIG. 17 is an implementation of a voltage booster used withimplementations of tunable resonant inductive coil systems.

DESCRIPTION

This disclosure, its aspects and implementations, are not limited to thespecific components, assembly procedures or method elements disclosedherein. Many additional components, assembly procedures and/or methodelements known in the art consistent with the intended tunable resonantinductive coil systems for wireless power transfer and near fieldcommunications and related methods will become apparent for use withparticular implementations from this disclosure. Accordingly, forexample, although particular implementations are disclosed, suchimplementations and implementing components may comprise any shape,size, style, type, model, version, measurement, concentration, material,quantity, method element, step, and/or the like as is known in the artfor such tunable resonant inductive coil systems for wireless powertransfer and near field communications and related methods, andimplementing components and methods, consistent with the intendedoperation and methods.

Referring to FIG. 2, a conventional resonant inductive coil system(system) 4 is shown. The system includes an electrical circuit 6 whichincludes an LC circuit (resonant circuit) 8. An alternating current (AC)voltage source 10 has a first terminal 12 and a second terminal 14. Afixed capacitor 16 is coupled in series with the AC voltage sourcethrough the first terminal. An inductor 18 which includes a coil 20 iscoupled in series with the fixed capacitor. A resistor 24 is coupled inseries with the coil. A number of lines 26 couple the various elementstogether. In the implementation shown the lines may be conductive lineson a semiconductor device, such as metallization, routing, and the like.A return line 22 electrically couples the resistor with the secondterminal of the AC voltage source.

A conventional series LCR resonant circuit such as that shown in FIG. 2has a resonant frequency which is dependent, at least in part, on thecapacitance of the fixed capacitor and the inductance of the inductor.The reactance of the capacitor at any given frequency is:

$X_{C} = \frac{1}{2\pi \; {fC}}$

where f is the frequency of the resonant circuit and C is thecapacitance of the fixed capacitor. The reactance of the inductor at anygiven frequency is:

X _(L)=2πfL

where f is the frequency of the resonant circuit and L is the inductanceof the coil/inductor. The resonant circuit is resonant when thereactances of the inductor and capacitor are equal, at:

X _(L) =X _(C)

which gives the resonant frequency as:

$f = \frac{1}{2\pi \sqrt{LC}}$

The total reactance of the LCR resonant circuit may be given as:

X=X _(L) −X _(C) +R

where R is the resistance of the resistor. Each of the reactances,including the total reactance, is measured in ohms.

Though not shown in the drawings, the various reactances and theresistance of the resistor may be plotted on the same graph wherereactance and resistance are plotted on the y coordinate and frequencyis plotted on the x coordinate. On such a graph the reactance of thecapacitor X_(C), reactance of the inductor X_(L), and resistance of theresistor when plotted together reveal that X_(C) is plotted as a curvethat decreases as f increases, X_(L) is a straight line which increasesas f increases, and the resistance of the resistor is a constant.Because X_(C) decreases with increasing frequency and X_(L) increaseswith increasing frequency, and because at the resonant frequencyX_(L)=X_(C), at frequencies below the resonant frequency the LCR circuitbehaves like a capacitor, at the resonant frequency it behaves like aresistor, and at frequencies above the resonant frequency it behaveslike an inductor.

For a conventional series tuned circuit, such as that described above,the current of the resonant circuit may be plotted versus frequency asshown in graph 2 of FIG. 1. Graph 2 plots current in milliamps versusfrequency in Hz on a logarithmic scale based on a Q value of the coil.The Q value is a coil quality factor, and current versus frequency isplotted for four different coils, having different Q values. It is seenthat the Q value affects the current, such that coils with higher Qvalues have higher peak currents. Coil 1 is shown having a Q value of 1and a peak current of about 10.0 mA, coil 2 is seen having a Q value of2 and a peak current of about 20.0 mA, coil 3 is seen having a Q valueof 5 and a peak current of about 50.0 mA, and coil 4 is seen having a Qvalue of 10 and a peak current of about 100.0 mA.

The Q value of the coil/system is inversely proportional to thebandwidth of the coil based on the relationship BW=F_(C)/Q, where BW isthe bandwidth and F_(C) is the resonant frequency of the system.Accordingly, as Q increases, the bandwidth decreases, so that a high Qresonant circuit has a narrow bandwidth relative to a low Q resonantcircuit. This increases the difficulty of tuning higher Q systems tohave the bandwidth centered at the driving frequency.

In both wireless power transfer (WPT) systems and near fieldcommunication (NFC) systems there generally exists a transfercoil/system (T_(X)) and a receiving coil/system (R_(X)). The T_(X) coilgenerates a magnetic field based in part on the peak current, and it isdesirable to have a higher peak current and to align the peak currentwith the resonant frequency of the system for the mosteffective/efficient transfer of power and/or data. Accordingly, in anideal system the bandwidth will be centered about the driving frequencyof the transferring system T_(X).

The resonant frequency of a system, however, may be affected by avariety of factors. Temperature and proximity to other magnetic objects,for example, may alter the resonant frequency of a system. As may beenvisioned, if the resonant frequency changes even slightly, especiallyfor higher Q systems, the current in the system can dramatically fall.As described above, both WPT and NFC systems work by transmittingmagnetic waves on a specific frequency from a transmitter to a receiver.The magnetic waves interact with a coil in the receiver to induce anelectric current. If the receiving coil is tuned so that its resonantfrequency matches the frequency of the magnetic waves, the current isamplified. If the receiver and transmitter are out of tune, however, thetransfer is inefficient. The receiver coil may still pick up a traceamount of current, but it will not be as amplified as desired.

The tuning of transferring and receiving coils/systems for wirelesspower transfer (WPT) is generally described in U.S. patent applicationSer. No. 14/843,819, filed Sep. 2, 2015, titled “Tunable/De-TunableWireless Power Resonator System and Related Methods,” listing as firstinventor Abdullah Ahmed (referred to hereinafter as the '819Application), now pending, the disclosure of which is entirelyincorporated herein by reference. Low frequency systems for wirelesspower transfer (WPT) using resonance may require coil antennas becausethe primary energy transport is in the B field rather than the E field.

Thus in any coil based charging or communication system the magneticfield generated is a function of the coil design and the peak currentdelivered into the coil by the driver. The coil is configured as aseries resonant load so that there is low resistance at peak resonance.The magnetic field strength of the transferring coil limits the transferfrom the transferring coil to the receiving coil through Faraday's lawwhich states that the rate of change of flux gives the electromotiveforce (voltage) resulting in the receiving coil. In addition to the Qvalue/factor, efficiency of the transfer between the primary(transferring) and secondary (receiving) coils is also affected by thediameter of each coil, the difference between the coil diameters(relative diameter), and the distance between the coils.

Accordingly, tuning of resonant circuits used for WPT or NFC allows thesystems to maintain high current in the coil (high Q) at the correctfrequency (BW center) to maintain efficient transfer. As describedabove, some such tuning methods and mechanisms are described in the '819Application. Because heat, nearby magnetic devices, and the like, canaffect resonance, tuning allows two coils (transferring and receivingcoils) to be brought back into resonance to ensure efficient transfer.

Some variable capacitors have been used for tuning conventional resonantinductive coil systems. In some cases, a switched capacitor or multipleswitched capacitors such as a capacitor bank, used to alter systemcapacitance by coupling and decoupling different fixed capacitors to andfrom the system using a plurality of switches, as desired, have beenused to achieve a variable capacitance of the overall system. Suchsystems by their nature involve “stepped” and not continuous capacitancechanges. Changing the capacitance of the system, as will be understoodfrom the above equations, will change the resonant frequency. Someswitched-capacitor systems use microelectromechanical (MEM) basedswitches. Varactor diodes (varicap diodes) have also been used toachieve variable capacitance for tuning of conventional resonantinductive coil systems, which does allow for continuous instead ofstepped tuning. Fixed trimmer capacitors have been used for tuningconventional resonant inductive coil systems pre-deployment, butnaturally once the system is fully packaged the trimmer capacitor(s) isnot re-tuned, so that there is no in-situ tuning during operation.

The capacitance of a parallel plate capacitor having two parallel platesboth of area A is given by the equation:

$C = {ɛ_{r}ɛ_{0}\frac{A}{d}}$

where C is the capacitance, A is the area of each parallel plate, d isthe distance between the plates, ∈₀ is the electric constant (about8.854×10⁻¹² F/m), and ∈_(r) is the dielectric constant of the materialbetween the plates. Variable capacitors in conventional resonantinductive coil systems have varied the area of the plates (such astrimmer capacitors, switching fixed capacitors in and out usingswitches, and MEMs based structures) or have varied the separationbetween plates (such as varactors or varicap diodes and air variablecapacitors). It has been observed that varactor diodes (which utilizeexponential variation) and switched capacitor systems lack stability insystems which utilize feedback to provide tuning of a resonant circuit.

WPT systems exist that include resonant inductive coil systems and areused, for example, to charge computing devices. Different chargingstandards exist. The type and configuration of interface circuit(resonant inductive coil circuit) used depends on the standard followed.One interface circuit standard is marketed under the name REZENCE byAlliance for Wireless Power (A4WP) of Altamonte Springs, Fla., and usesa frequency of 6.78 MHz. Another interface standard is marketed underthe name QI by Wireless Power Consortium of Piscataway, N.J. (thoughsome products marketed under the name QI use magnetic coupling orelectromagnetic induction alone without resonant inductive charging).Resonant charging using resonant inductive coil systems have been foundto have an increased charging range over conventional inductivecharging, as will be discussed to some extent hereafter. Conventionalelectromagnetic induction charging systems that do not use resonantinductive charging also require coil alignment to a greater degree thanis needed for resonant inductive coil system charging.

Referring now to FIG. 3, in implementations a tunable resonant inductivecoil system (system) 28 includes an electrical circuit 30 which forms anLC circuit (resonant circuit) 32. An alternating current (AC) voltagesource 34 has a first terminal 36 and a second terminal 38. A variablecapacitor 40 is coupled in series with the first terminal of the ACvoltage source. While various types of variable capacitors could beused, such as those described above, in the implementation of FIG. 3 thevariable capacitor is a barium strontium titanate (BST) variablecapacitor 42. The FIG. 3 implementation is a single ended configuration.

As described above, the capacitance of a variable capacitor may bevaried by varying the capacitative area and/or the separation betweencapacitative plates. With a BST variable capacitor the capacitance maybe varied by varying the ∈_(r) value or, in other words, by varying thedielectric constant of the material between the capacitative plates.

Referring still to FIG. 3, an inductor 44 is coupled in series with thevariable capacitor. The inductor shown is a coil 46. A number of lines54, such as metallization and routing, are used to couple the variouselements of the electrical circuit together. A return line 50 couplesthe coil with a ground 52, and the second terminal of the AC voltagesource is also coupled with ground. An entire length 48 of the coil isdepicted in FIG. 3.

FIG. 4 shows a tunable resonant inductive coil system (system) 56 thatis in some ways similar to system 28 but which adds an additionalvariable capacitor in series with the inductor between the inductor anda ground on the return line. System 56 includes an electrical circuit 58which forms an LC circuit (resonant circuit) 60. An alternating current(AC) voltage source 62 includes a first terminal 64 and a secondterminal 66. A first variable capacitor 68 is coupled in series with thefirst terminal of the AC voltage source. The first variable capacitormay be any type of variable capacitor, as those described above, but inthe representative example is a BST variable capacitor 70. The FIG. 4implementation is a double ended configuration.

An inductor 76, which comprises a coil 78, is coupled in series with thefirst variable capacitor. An entire length 80 of the coil is depicted inFIG. 4. A second variable capacitor 72 is coupled in series with thecoil on a return line 82 between the coil and a ground 84. Although thesecond variable capacitor may be any of a variety of capacitor typesdisclosed above, in the implementation shown it is a BST variablecapacitor 74. A number of lines 86, which may include metallization andother conductive elements, couple the various elements of the electricalcircuit.

FIG. 5 shows a tunable resonant inductive coil system (system) 88 whichis similar to system 28 except that an additional fixed (non-variable)capacitor is placed in series with the variable capacitor between thevariable capacitor and the coil. System 88 includes an electricalcircuit 90 which forms an LC circuit (resonant circuit) 92. Analternating current (AC) voltage source 94 includes a first terminal 96and a second terminal 98. A variable capacitor 100 is coupled in serieswith the first terminal of the AC voltage source. The variable capacitormay include any of a variety of capacitor types, such as those disclosedabove, but in the implementation shown is a BST variable capacitor 102.A fixed capacitor 104 is coupled in series with the variable capacitorbetween the variable capacitor and an inductor 106. The inductorincludes a coil 108, and an entire length 110 of the coil is depicted. Areturn line 112 couples the coil with a ground 114, and a number oflines 116 (including the return line), which may include metallizationor other conductive materials, electrically couple the various elementsof the electrical circuit. The FIG. 5 implementation is a single endedconfiguration.

FIG. 6 shows a tunable resonant inductive coil system (system) 118 thatis similar to system 56 except including additional fixed capacitors.System 118 includes an electrical circuit 120 that forms an LC circuit(resonant circuit) 122. An AC voltage source 124 has a first terminal126 and a second terminal 128. A first variable capacitor 130, which inthe implementation shown is a BST variable capacitor 132, is coupled inseries with the first terminal. A first fixed capacitor 134 is coupledin series with the first variable capacitor. An inductor 136, whichincludes a coil 138, is coupled in series with the first fixedcapacitor. An entire length 140 of the coil is depicted. A second fixedcapacitor 148 is coupled in series with the coil on a return line 142,and a second variable capacitor 144, which is a BST variable capacitor146, is coupled in series with the second fixed capacitor on the returnline. The return line couples the second variable capacitor with thesecond terminal and with a ground 150. A number of lines 152 which mayinclude metallization, routing, and/or the like (and which include thereturn line), electrically couple the various elements. The FIG. 6implementation is a double ended configuration.

FIG. 7 shows a tunable resonant inductive coil system (system) 154 thatis similar to system 28 except that a fixed capacitor is coupled inparallel (shunt) with the variable capacitor. System 154 includes anelectrical circuit 156 that forms an LC circuit (resonant circuit) 158.An AC voltage source 160 has a first terminal 162 and a second terminal164. A variable capacitor 166 is coupled in series with the firstterminal of the AC voltage source. The variable capacitor in theimplementation shown is a BST variable capacitor 168 though, as withother implementations, could include another type of variable capacitor.A fixed capacitor 170 is coupled in parallel (shunt) with the BSTvariable capacitor. An inductor 172, which in the implementation shownis a coil 174, is coupled in series with both the BST variable capacitorand the fixed capacitor. An entire length 176 of the coil is depicted. Areturn line 178 electrically couples the coil with a ground 180, and thesecond terminal of the AC voltage source is also coupled with theground. Lines 182, such as metallization, traces, routes, etc. (andincluding the return line), electrically couple the various elementstogether. The FIG. 7 implementation is a single ended configuration.

FIG. 8 shows a tunable resonant inductive coil system (system) 184 thatis similar to system 56 except that two fixed capacitors are added, onein parallel (shunt) with each variable capacitor. System 184 includes anelectrical circuit 186 that forms an LC circuit (resonant circuit) 188.An AC voltage source 200 has a first terminal 202 and a second terminal204. A first variable capacitor 206, which is a BST variable capacitor208 in the implementation shown, is coupled in series with the firstterminal of the AC voltage source. A first fixed capacitor 210 iscoupled in parallel (shunt) with the first variable capacitor. Aninductor 212, which in the implementation shown is a coil 214, iscoupled in series with both the first variable capacitor and the firstfixed capacitor. An entire length 216 of the coil is depicted. The FIG.8 implementation is a double ended configuration.

On a return line 218, which couples the coil with a ground 226, a secondvariable capacitor 220, which in the implementation shown is a BSTvariable capacitor 222, is coupled in series with the coil. A secondfixed capacitor 224 is coupled in parallel (shunt) with the secondvariable capacitor and in series with the coil. A number of lines 228,which may include metallization, traces, routes, etc. (and including thereturn line), electrically couple the various elements together.

FIG. 9 shows a representative of a coil implementation. Coil 230 islocated on/near a surface of a battery 232 of a mobile computing device,such as a smart phone. The coil 230 could be used as the coil for any ofthe systems described above. As the coil in all of the implementationsmay be used to communicate data and/or energy via resonant magneticcoupling, the coil in various implementations may also be an antenna.FIG. 9 illustrates only one representative example of a coil, and inother implementations the coil could be implemented on a motherboard orprinted circuit board (PCB), within one or more semiconductor devices orpackages of a computing device, and so forth.

FIG. 10 shows a representative example of two coils or antennas in closeproximity with one another but not perfectly aligned (i.e., slightlyoffset), though nevertheless configured to resonate at the samefrequency. The resonant coils 234 include a first coil 236 and a secondcoil 238. The two coils could be representative of different computingdevices. For example, one coil could be implemented in a smart phone, asdescribed above, and another could be implemented in a charging station,such as to charge the phone using WPT. One coil could be implemented ina smart phone and another could be implemented in a terminal or paymentkiosk at a store, such as for payment via NFC. One coil could be in onemobile device and the other could be in another mobile device, to allowdata transfer between mobile devices (such as between two mobilephones). The coils are not limited to mobile phones and may beimplemented in tablets, laptops, desktop computers, and soforth—essentially in any computing device.

FIG. 11 shows a graph 240 which is representative of the resonantcoupling of the two coils of FIG. 10. Two coils similar or identical tothose shown in FIG. 10 were modeled using modeling software, with someoffset from perfect alignment as described, and resonant coupling duringmagnetic resonance between antennas (in other words, the couplingcoefficient S₂₁ in dB) was determined for various separation distancesas a function of frequency. The modeling software used was a softwaremarketed under the name Advanced Design Systems (ADS) by KeysightTechnologies Inc. of Santa Rosa, Calif. FIG. 11 is a graph which showsthe resonant coupling that was modeled when the antennas/coils were at adistance of 3.0 cm from one another. As can be seen the coupling wassimulated between frequencies of 2 and 20 MHz. The two points m1 and m2on the graph represent values that were chosen to be somewhat close tothe target frequencies for WPT and NFC. They are not exactly similar tothe target frequencies but are relatively close, with m1 being 6.276 MHz(instead of 6.25 MHz or 6.78 MHz) and m2 being 13.62 MHz (instead of13.25 MHz or 13.56 MHz).

The square dots on the graph of FIG. 11 represent discrete modeledvalues for the resonant coupling and the curve represents a fitted curveusing the discrete values. Table 1, below, shows the couplingcoefficient S₂₁ in dB at the 6.276 MHz and 13.62 MHz values forseparation distances of 0.5 cm, 1.0 cm, 1.5 cm, 2.0 cm, 2.5 cm, and 3.0cm.

TABLE 1 coupling coefficient S21 for various separation distancesSeparation S₂₁ at 6.276 MHz S₂₁ at 13.62 MHz (cm) (dB) (dB) 0.5 −12.948−12.392 1.0 −16.207 −16.053 1.5 −19.042 −19.102 2.0 −21.562 −21.735 2.5−23.847 −24.085 3.0 −25.944 −26.244As has been described above, magnetic resonant coupling allows fortransfer of energy/data over greater distances than traditionalinductive coupling, and the modeled coupling coefficients of Table 1reflect this, showing relatively significant coupling even at distancesof 3.0 cm between the paired coils.

FIG. 12 shows a tunable resonant inductive coil system (system) 242which is based upon modeling one of the above modeled coils of FIGS.10-11 as a load. System 242 includes an electrical circuit 244 whichincludes an LCR circuit (resonant circuit) (LC circuit) 246 (seen at theright side of the drawing and including L0, C0 and R0). An AC voltagesource 248 has a first terminal 250 and a second terminal 252. Thesecond terminal is coupled with a ground 274. A variable capacitor 270(which is labeled C6 having a capacitance of c=Csm), which in theimplementation shown is a BST variable capacitor 272 (but which in otherimplementations could include other types of capacitors describedabove), is coupled with the first terminal of the AC voltage source. Aninductor 256, which includes a coil 258, is coupled in series with thevariable capacitor, and an entire length 260 of the coil 258 isdepicted. A return line 262 couples the coil with a ground 274.

A control element 277 is also included, which controls the variablecapacitor. In implementations the BST variable capacitor may be a BSTvariable capacitor marketed under the name Passive Tunable IntegratedCircuit (PTIC) by On Semiconductor of Phoenix, Ariz. For example, theBST variable capacitor could have a configuration similar to that shownin Appendix A (“TCP-4182UB—8.2 pF Passive Tunable Integrated Circuits(PTIC)”), the entire disclosure of which is incorporated herein byreference, except that while the BST variable capacitor of Appendix A isconfigured to have a tuning range from between 1 pF and 10 pF, the BSTvariable capacitor of FIG. 12 may be configured to have a 3:1 tuningratio from 25 pF to 75 pF. The BST variable capacitor may be in a largerpackage than that shown in Appendix A, and could for example fit in anexisting 12 pillar frame and have a bias RC of about 800 Hz. Otheradjustments may include a different balance between bias resistor versuscapacitor area to achieve an appropriate RC charging constant,differences in the design of vias to optimize power flows, and the like.

The control element 277 controls the BST variable capacitor to vary itscapacitance. A representative example of a control element is given inAppendix B (“TCC-103—Three-Output PTIC Control IC”), which is entirelyincorporated herein by reference. The TCC-103 control element describedin Appendix B, however, has some additional functionality than what isneeded for system 242, and so an alternative control element using adiode based voltage multiplier may be used instead, as will be describedhereafter. The control element sends one or more control signals to theBST variable capacitor to vary the capacitance of the BST variablecapacitor.

Referring still to FIG. 12, a switch 264 is coupled with the BSTvariable capacitor. The switch in the implementation shown is a magneticswitch (switch) 266 which is controlled by inductor (coil) 268. Switch266 in implementations is RF power capable. A direct current (DC)voltage source 254 has a first terminal coupled with the coil 268 and asecond terminal coupled with a ground 274. The DC voltage source variesan inductance of the coil 268 which in turn operates the switch. Thecoil 268 is coupled with a ground 274.

When the switch is open, a fixed capacitor C9 is electrically isolatedfrom the BST variable capacitor and the resonant circuit, and when theswitch is closed fixed capacitor C9 is electrically coupled with the BSTvariable capacitor and the resonant circuit so that the fixed capacitorC9 is in series with the BST variable capacitor and in parallel with thecoil 258. The fixed capacitor is coupled with a ground 274.

A number of additional passive electrical components 276 are included insystem 242 in addition to capacitor C9, including resistor R2 couplingthe first terminal of the AC voltage source with the BST variablecapacitor, capacitor C0 coupling the BST variable capacitor with aground 274 (and part of the LCR circuit), and resistor R0 coupling theBST variable capacitor with the coil 258 (and also part of the LCRcircuit). The value of each component in the circuit is given—forexample resistor R2 is given a value of “850.00 m” by which is meant 850milliohms. C9 has a value of 150 pF, C0 has a value of 7 pF, resistor R0has a value of 900 milliohms, and coil 258 (L0) has a value of 3.2microHenries. In other implementations the switch and fixed capacitor C9may be excluded, and in other implementations one or more of theadditional passive electrical components 276 may be excluded. In otherimplementations different or additional passive electrical components276 may be added.

Referring still to FIG. 12, a voltage at the first terminal of the ACvoltage source is represented by V0 and at the V0=1 setting is seen tohave a frequency of 6.2 MHz (though this is short for 6.25 MHz) while atV0=0 it will have a frequency of 13.25 MHz. Switch W0 is seen to have anopen setting 0 which corresponds with the 13.25 MHz frequency and aclosed setting 1 which corresponds with the 6.25 MHz frequency. The DCvoltage source has next to it the phrase “vdc=SwCtrllforLowF” indicatingthat the switch is switched to the closed position for the 6.25 MHzfrequency (Low Frequency).

Other elements may be included which are not shown in the simplifieddiagram of FIG. 12 for ease of viewing the elements that are present.There may be, for instance, one or more controllers controlling the ACand DC voltage sources, and other passive components as has beendescribed above. The operation of system 242 involves adjusting theelectrical circuit between two configurations. In a first configurationthe DC voltage source would be controlled (or turned off) so that theswitch is closed. This corresponds with the 6.25 MHz frequency. Thecontrol element 277 will, correspondingly, adjust the capacitance of theBST variable capacitor. For the 6.25 MHz setting the capacitance of theBST variable capacitor, which has been described as ranging from 25-75pF, is adjusted to 42.0 pF. The adjustment of the capacitance of the BSTvariable capacitor and the closing of the switch 264 “correspond” withone another in the sense that they are both done in order to achieve theadjusting of the resonant frequency of the resonant circuit 246 to 6.25MHz, but neither adjustment is necessarily caused by (or is a directresult of) the other (though in other implementations such could be thecase). Nevertheless, one or more electrical signals sent from acontroller to control the DC voltage source to operate the switch andone or more control signals sent from the control element 277 may besynchronized in time to occur at the same, or about the same, time toachieve effective adjustment of the electrical circuit from oneconfiguration to the other.

The above configuration, referred to as a “first configuration,” whereinthe resonant frequency of the resonant circuit 246 is adjusted to 6.25MHz, is configured to be used for wireless power transfer (WPT). It isnoted that the entire length 260 of antenna/coil 258 is used for thewireless power transfer.

To switch from the first configuration to a second configurationconfigured for near field communication (NFC), the DC voltage source iscontrolled to open the switch 264 and the control element 277correspondingly sends one or more control signals to the BST variablecapacitor to adjust the capacitance of the BST variable capacitor to32.3 pF. The opening of the switch and control of the BST variablecapacitor “correspond” in the same way as described earlier, and neitheris necessarily controlled by or caused by the other (though in otherimplementations such could be the case).

In this second configuration the resonant frequency of the resonantcircuit 246 switches to 13.25 MHz, which is a frequency configured fornear field communication (NFC). It is noted that the entire length 260of the antenna/coil 258 is used for the near field communication.Accordingly, in both wireless power transfer and in near fieldcommunication (or in other words at both the 6.25 MHz and at the 13.25MHz frequencies) the entire length of the coil 258 is used. Similarly,if any of the other system configurations described previously are usedfor a tunable resonant inductive coil system the entire length of thecoil of the respective resonant circuit may be used for both wirelesspower transfer and near field communication.

Using the system 242, a computing device may, using a single antenna orcoil, (1) charge another device or receive a charge from another deviceusing a WPT frequency of 6.25 MHz, and (2) communicate with anotherdevice (or the same device) using a near field communication (NFC)frequency of 13.25 MHz, by switching the resonant frequency of theresonant circuit of which the coil is a part. The inclusion of the BSTvariable capacitor in a matching filter allows the resonant frequency ofthe resonant circuit to be reconfigured for two different standards.

Although frequencies of 6.25 MHz for WPT and 13.25 MHz for NFC aredescribed above, the practitioner of ordinary skill in the art willunderstand the variations that may be made for components of the system242 in order to configure the system so that the lower frequency is 6.78MHz to match the A4WP standard referenced above and a higher frequencyof 13.56 MHz to match the common NFC frequency. The reader will noticethat the lower frequency is half the higher frequency, so that analternate embodiment could utilize one half of the coil 258 for oneresonant frequency and the full coil 258 for another frequency, but oneadvantage of using the full coil as described above for both frequenciesis that the signal strength is not reduced by using less of the coil.Another alternative is to use two antennas or coils but some advantagesof using a single coil for both frequencies are: (1) it allows for onelarger antenna instead of two smaller antennas which results in agreater signal strength for both signals; (2) it allows for moreefficient use of space in/on a computing device, where sometimes it isprohibitive or less desirable to have to use space for two antennasinstead of one antenna, and; (3) it allows for lower production cost dueto only needing to have one antenna.

Not shown in FIG. 12, but included, may be a near field communication(NFC) chip. The NFC chip may control one or more aspects of the nearfield communications transmitted from a computing device and/or receivedby a computing device. The NFC chip could be damaged by wireless powertransfer signals but, in implementations, the NFC chip is protected fromWPT signals because adjusting the resonant frequency of the resonantcircuit to the NFC frequency significantly detunes the resonant circuitfrom any nearby wireless charging source so that power is not received(or very little is received) at the NFC frequency. This effectiverejection by the system/filter of WPT signals at NFC frequencies mayallow for the dual use of WPT and NFC frequencies with a single antenna.The NFC chip and/or the electrical circuit may further have a notchfilter for protection. In implementations the NFC chip is not activateduntil the electrical circuit is at the NFC resonant frequency—in otherwords the NFC chip may not search for NFC frequencies being transmittedto it until the system is set to the NFC configuration.

FIG. 13 shows a graph 278 which represents an optimizing of the BSTvariable capacitor at two different frequencies for the NFC and WPTfunctionalities. The specific frequencies chosen in the representativeexample are 6.25 MHz and 13.56 MHz, though as indicated above thefrequencies may be chosen as desired by a user, and for example may beset instead to 6.78 MHz and 13.56 MHz. Using the representative examplesof FIG. 13, the capacitance of the variable capacitor may be chosen sothat the peak current is achieved at the desired frequencies, and usingthe model the capacitances of 42.0 pF and 32.3 pF were found to achievepeak currents at the frequencies of 6.25 MHz and 13.56 MHz.

The capacitances of 42.0 pF and 32.3 pF are used when the system isexactly at 6.25 MHz and 13.56 MHz, respectively, but as described abovethe resonant frequency of the resonant circuit may be affected by manyvariables, such as heat, nearby magnetic devices or components, and thelike. Accordingly, the target values of 6.25 MHz and 13.56 MHz may notachieve the optimum transfer efficiency between the transferring andreceiving coil given different environmental conditions. Once the system242 is in either the first or second configuration, however, thecapacitance of the variable capacitor can then be adjusted all the waydown to 25 pF or all the way up to 75 pF to fine tune the frequency andagain achieve optimum resonant coupling. Thus, if the coil of thetransmitter or receiver coil moves away from resonance (effectiveinductance L increases or decreases) the capacitance may be varied(decreased or increased, respectively) to bring the coil back intoresonance.

As can be seen in FIG. 14, graph 280 shows that while the capacitance of42.0 pF achieves the frequency of 6.25 MHz (corresponding with the V1voltage), once the system is in that configuration with the switchclosed, the variable capacitor may be further adjusted all the way downto 25.0 pF to increase the resonant frequency to 6.525 MHz(corresponding with a voltage magnitude of 11.3693 V). The variablecapacitor may also be adjusted all the way up to 75.0 pF to decrease theresonant frequency to 5.8 MHz (corresponding with a voltage magnitude of26.877 V). The variable capacitor may also be adjusted to any value inbetween to achieve any resonant frequency between 5.8 MHz and 6.525 MHz.In other implementations the variable capacitor and/or the system may bealtered (such as by using the system 242 with a variable capacitorhaving a maximum capacitance above 75.0 pF) in order to achieve a rangeof 5.8 MHz to 7.525 MHz or some other range including the A4WP standardof 6.78 MHz).

FIG. 15 shows graph 282 which shows that while the capacitance of 32.3pF may achieve the frequency of 13.25 MHz (or 13.56 MHz) (correspondingwith the V1 voltage), once the system is in that configuration with theswitch open, the variable capacitor may be further adjusted all the waydown to 25.0 pF to increase the resonant frequency to 14.85 MHz(corresponding with a voltage magnitude of 27.7978 V). The variablecapacitor may also be adjusted all the way up to 75.0 pF to decrease theresonant frequency to 9.625 MHz (corresponding with a voltage magnitudeof 61.3356 V). The variable capacitor may also be adjusted to any valuein between to achieve any resonant frequency between 9.625 MHz and 14.85MHz.

Naturally, the above representative examples are only illustrative ofthe various frequency shifts that can be accomplished to achieve bothNFC and WPT frequencies and to fine tune around those frequencies toadjust for environmental changes and the like that may affect resonantfrequencies. Thus, a system such as system 242 may be configured tooperate at a first frequency of 6.78 MHz for WPT and at a secondfrequency of 13.56 for NFC, and to be further adjustable using avariable capacitor, such as a BST variable capacitor as describedherein, to fine tune around either of those frequencies when thetransmission and/or receiving antenna(s) becomes out of tune with thetarget frequency.

The system 242 and any similar system(s) described herein may beimplemented at either antenna, transmission or receiving, or both, andmay include feedback so that one system adjusts the resonant frequencyof its resonant circuit in order to match the other system's resonantfrequency, or vice versa, or both systems may be adjustable to match theother.

As has been described above, the TCC-103 controller is a representativeexample of a controller that could be used as the control element 277,but may not be needed. System 242 and similar systems may have veryrelaxed timing constraints compared to what the TCC-103 controller iscapable of offering, and it may be desirable to have a differentinterface (perhaps a more complex interface) than the TCC-103 interface.A voltage booster instead may be implemented with an eightdiode/capacitor multiplier using eight 1 nF 10 V capacitors and fourdual hot carrier diodes such as those shown in Appendix C“MMBD452LT1G—Dual Hot-Carrier Diodes”), the disclosure of which isincorporated entirely herein by reference.

FIG. 17 shows a circuit diagram 286 of a voltage booster 288 which maybe used as a control element 277. The voltage booster includes aCockcroft Walton multiplier 290. A first voltage source 292 (representedas a sine wave input) is coupled with ground 296 and is a first inputfor a double pole double throw (DPDT) analog switch (switch) 301. Asecond voltage source 294 (represented as a step wave input) is coupledwith ground and is a second input for switch 301. Switch 301 furtherincludes a number of switches 298 and inductors 300, not all of whichare numbered in FIG. 17 but all of which are enclosed within the dottedoutline of switch 301.

The Cockroft Walton multiplier 290 is executed using a number ofcapacitors 302 and diodes 304. Diode pairs may be implemented using dualhot carrier diodes as described above in Appendix C, so that theleftmost diode 304 in FIG. 17 and the next closest diode are included ina single dual hot carrier diode, and each next pair is likewise includedin a single dual hot carrier diode, thus the eight Schottky barrierdiodes are implemented using four dual hot carrier diodes. A number ofresistors 306 are also included, and the rightmost resistor 306 is forbiasing of the variable capacitor.

In this way the DPDT analog switch may be used to chop aprogrammable/variable DC input available from the variable capacitor(such as a PMIC as described above) which may range from 0 V to 3.6 V,for example. In the representative example shown in FIG. 17 the DPDTanalog switch may have a frequency between 100-200 kHz, the capacitorsforming the Cockroft Walton multiplier may be 10V 1 nF capacitors, therightmost resistor (leakage resistor) 307 may have a resistance of 1Megaohm, the next resistor to the left may have a resistance of 1Kiloohm, and the capacitor between that resistor and ground 296 may havea capacitance of 10 nF. Other configurations are possible, and this isonly one representative example.

As seen above, a system 242 or similar system may be implemented using aPTIC variable capacitor, a voltage booster, and other elements shown inFIG. 12 for a matching filter. Additional components may be utilized aswill be understood by the practitioner of ordinary skill in the art tomodify the system.

FIG. 16 is a graph 284 plotting voltage versus time of the voltagebooster 288 showing the input voltage (dotted line) and output voltage(solid line), revealing how increases in input voltage (such as from 0to 10 milliseconds and from 35 to 50 milliseconds) correspond with sharprises in the output voltage. The rising and stabilization of the outputvoltage is seen to lag the rising and stabilization of the input voltageby about 3-4 ms. At a time of 26.9671 ms the input voltage is 2.0 V andthe output voltage is 8.4716 V, showing more than a quadrupling of thevoltage.

In implementations a control element 277 may include the above describedvoltage booster which may operate on less current than a TCC-103controller. The voltage booster (or boost converter) may be coupled witha GND pin (ground pin), a VBAT pin (coupled with a battery or powersupply), and a VHV pin (high voltage pin). Control logic of the controlelement may include a Vout+ and Vout− elements and a StartPositionHorL(for starting position high or low) logic element(s) or a mobileindustry processor interface (MIPI) clock data plus a triggering elementor logic. The control element may include a clock generator to generatewaves for boost and control timing. The control element may also includea one-time programmable (OTP) element for trim, or in other words to setthe initial target values for the variable capacitor for the target WPTfrequency and the target NFC frequency within the overall frequencyrange of the variable capacitor. An output pin OUT may be included. Thecontrol element, when implemented using the described voltage booster,may include less inputs than a TCC-103 controller and may be slower. Adigital power VIO pin and additional ground GND pin may also beincluded. In total the control element 277 may have 9 pins including 2grounds in a 3×3 array of 0.4 mm redistribution layer (RDL) bumps.

A control element as described above may be implemented using a die sizeof 1.28 mm by 1.28 mm (including a 1.2 mm by 1.2 mm die plus 80 μmscribes. The control signal(s) from the control element to the variablecapacitor in implementations includes one or more DC control voltages.The above example using a voltage booster is only one of many examplesof what could be used to control the variable capacitor.

The coils/antennas described herein may be implemented using a varietyof elements. In implementations the coils/antennas may be formed of aconductive metal such as copper, gold, silver, any alloy of the same,and the like. Referring to FIG. 10 a representative coil formed ofcopper could be formed using (or using about) 1 ounce of copper with atrack width (W) of 400 microns, a track separation (d) of 200 microns, atrack thickness (t) of 35 microns (or 34.79 microns), an antenna outsideX dimension (AntL) of 50 mm, an antenna outside Y dimension (AntW) of 50mm, and a total of 3.75 complete turns with 15 total sides. The coil mayhave a total inductance of 3.27 microHenries and a total resistance of0.87 ohms at 20 degrees Celsius. Different specifications may be used ifthe material is gold, silver, an alloy, or some other element, toachieve the desired inductance, resistivity, and other characteristics.The coil(s) of any of the systems described herein may be designedtaking self and mutual inductive effects (effects between paired coilsfrom different computing/charging devices) and other factors intoconsideration. The above design parameters are based on a copperresistivity of 1.68×10⁻⁸ ohm meters and a conductivity of 8.96×10⁷ S/m.

The above described systems thus include an antenna, the ability toshift between one high frequency and one low frequency for NFC and WPT,fine tuning capability with continuous tuning control for fine tuningwhen coils are not optimally resonant with one another at the target NFCand WPT frequencies, and may include AC/DC power conversion plusprotection of an NFC chip, as described above. Protection of theantenna, to shield it from undesirable interference from some elementsof the computing device itself or from other devices, may also beincluded in some implementations. The fine tuning of the frequency atWPT levels allows optimization of the power transfer efficiency todesirable levels. When the system is configured for WPT it forms amatching coil wireless charging interface circuit.

The above elements thus allow the design of tunable radio frequency (RF)devices, such as antennas in cell phones, tablets, laptops, terminals,kiosks, payment devices, and other computing devices, for the use ofnear field communication (NFC) and wireless power transfer (WPT). TheNFC capability may allow, by non-limiting example, data exchange,payment capabilities, and other services/capabilities that may be usedat short range (such as around 3.0 cm or closer).

While system 242 shows a more specific implementation of a tunableresonant inductive coil system, such a layout may be modified to use anyof the configurations shown in FIGS. 3-8 for different implementationsof tunable resonant inductive coil systems. Any of the tunable resonantinductive coil systems described herein may be used for either atransmission coil, a receiving coil, or both.

In any of the tunable resonant inductive coil systems described hereinthe coil could be replaced by a non-coil B-field generator. As indicatedabove, non-BST capacitors may be used for the variable capacitor(s) inimplementations, and may include any capacitor that varies capacitancebased on receipt of one or more electrical control signals. Any of theBST variable capacitors described herein may include doped BSTdielectrics.

The use of BST variable capacitors may have the following advantages:(1) continuously variable capacitance; (2) stable feedback (i.e., whenadjustment is needed to correct dissonance detected between primary andsecondary coils); (3) good linearity performance (generally linearbehavior of reactance vs. frequency for resonant circuit above theresonant frequency); (4) high Q values; (5) small size; (6)manufacturable using inexpensive, repeatable, and scalable semiconductorprocesses; (7) good tuning range; (8) good tuning speed (capacitancevariation rate); (9) electric control (Vbias adjustment which may begenerated from many sources); (10) excellent matching due to repeatablesemiconductor processes (including simultaneous fabrication of dualstructures); (11) low cost; (12) good range of capacitor sizes suitablefor multiple coil configurations, and; (13) common assembly methods suchas wafer level chip scale packaging (WLCSP) soldering flows.

While common WPT frequencies are discussed above, such as 6.78 MHz ofthe A4WP standard which is common in the mobile communications industry,the above elements and methods may also be used to create custom systemsusing different frequencies such as in the medical, automotive, andother industrial and consumer industries. The matching of resonantfrequencies for RF power transfer systems may reduce reflective powerloss at antenna interfaces.

Any of the tunable resonant inductive coil systems disclosed herein mayinclude additional passive electrical components. Any of the fixedcapacitors described herein may include any type of fixed capacitor. Thespecific circuit diagrams given herein are only representative examplesand various other configurations are possible for other implementationsof tunable resonant inductive coil systems.

Methods of use of any of the tunable resonant inductive coil systemsdescribed herein may include providing any of the electrical circuitsdescribed herein and, in response to the electrical circuit receiving acontrol signal (such as from control element 277 or the like) at thevariable capacitor, adjusting a capacitance of the variable capacitor toadjust a resonant frequency of a resonant circuit of the electricalcircuit between a first resonant frequency configured for wireless powertransfer (WPT) (which may or may not be 6.78 MHz or 6.25 MHz) and asecond resonant frequency used for near field communication (NFC) (whichmay or may not be 13.25 MHz or 13.56 MHz). The methods, as indicatedabove, may include not using only part of the coil/antenna for eachfrequency, but using an entire length of the coil, or in other words theentire coil, for both the NFC frequency and the WPT frequency. Themethod of switching between NFC and WPT target frequencies may includeopening/closing a switch of the electrical circuit, as described abovewith reference to FIG. 12, which may electrically couple a fixedcapacitor in parallel with the coil. The method may include tuning thecapacitance of the variable capacitor to adjust the resonant frequency(either or both of the first and/or second resonant frequency) inresponse to the resonant circuit being out of resonance with a secondresonant circuit (having a paired secondary coil) of a computing device.The method may include tuning the NFC resonant frequency within a rangeof 5.8 MHz and 6.525 MHz (though other implementations may includetuning within a range that includes the A4WP standard of 6.78 MHz, suchas a range from 5.800 MHz to 7.525 MHz, such as by using a variablecapacitor with maximum capacitance above 75.0 pF) and tuning the WPTresonant frequency within a range of 9.625 MHz and 14.850 MHz. Themethod may include protecting an NFC chip, described above, from damagefrom a power source by detuning the resonant circuit from the powersource signal at NFC frequencies.

In places where the description above refers to particularimplementations of tunable resonant inductive coil systems for wirelesspower transfer and near field communication and related methods andimplementing components, sub-components, methods and sub-methods, itshould be readily apparent that a number of modifications may be madewithout departing from the spirit thereof and that theseimplementations, implementing components, sub-components, methods andsub-methods may be applied to other tunable resonant inductive coilsystems for wireless power transfer and near field communication andrelated methods.

What is claimed is:
 1. A tunable resonant inductive coil system,comprising: an electrical circuit comprising an alternating current (AC)voltage source, a barium strontium titanate (BST) variable capacitorcoupled in series with a first terminal of the AC voltage source, a coilcoupled in series with the BST variable capacitor, and a return linecoupling the coil with one of a second terminal of the AC voltage sourceand a ground; wherein the electrical circuit forms an LC circuit(resonant circuit); wherein the electrical circuit is configured to, inresponse to receiving one or more electrical signals, adjust between afirst configuration and a second configuration, wherein in the firstconfiguration the resonant circuit has a first resonant frequencyconfigured to be used for wireless power transfer and in the secondconfiguration the resonant circuit has a second resonant frequencyconfigured to be used for near field communication (NFC), wherein anentire length of the coil is used for both the first resonant frequencyand the second resonant frequency, and; wherein adjusting the electricalcircuit between the first configuration and the second configurationcomprises varying a capacitance of the BST variable capacitor inresponse to receiving a control signal at the BST variable capacitor. 2.The system of claim 1, wherein the coil comprises an antenna.
 3. Thesystem of claim 1, wherein adjusting the electrical circuit between thefirst configuration and the second configuration comprises one ofopening and closing a switch of the electrical circuit to one ofelectrically couple a fixed capacitor with the resonant circuit andelectrically isolate the fixed capacitor from the resonant circuit. 4.The system of claim 1, further comprising a second BST variablecapacitor coupled in series with the coil on the return line.
 5. Thesystem of claim 4, further comprising a fixed capacitor coupled inparallel with the second BST variable capacitor and coupled in seriesbetween the coil and one of the second terminal of the AC voltage sourceand the ground.
 6. The system of claim 1, further comprising a fixedcapacitor coupled in series between the first terminal of the AC voltagesource and the coil.
 7. The system of claim 1, further comprising afixed capacitor coupled in series between the coil and one of the secondterminal of the AC voltage source and the ground.
 8. The system of claim1, further comprising a fixed capacitor coupled in parallel with the BSTvariable capacitor and in series between the first terminal of the ACvoltage source and the coil.
 9. The system of claim 1, wherein the firstresonant frequency is between 5.800 MHz and 7.525 MHz and wherein thesecond resonant frequency is between 9.625 MHz and 14.850 MHz.
 10. Atunable resonant inductive coil system, consisting of: an electricalcircuit consisting of an alternating current (AC) voltage source; one ormore barium strontium titanate (BST) variable capacitors, at least oneof the one or more BST variable capacitors coupled in series with afirst terminal of the AC voltage source; a coil coupled in series withthe first terminal of the AC voltage source; a return line coupling thecoil with one of a second terminal of the AC voltage source and aground; a switch coupled with the AC voltage source; a direct current(DC) voltage source controlling the switch, and; one or more additionalpassive electrical components; wherein the electrical circuit forms anLC circuit (resonant circuit); wherein the electrical circuit isconfigured to, in response to receiving one or more electrical signals,adjust between a first configuration and a second configuration by oneof opening and closing the switch and by correspondingly varying acapacitance of the one or more BST variable capacitors, wherein in thefirst configuration the resonant circuit has a first resonant frequencyconfigured to be used for wireless power transfer and in the secondconfiguration the resonant circuit has a second resonant frequencyconfigured to be used for near field communication (NFC), and; whereinan entire length of the coil is used for both the first resonantfrequency and the second resonant frequency.
 11. A method of use of atunable resonant inductive coil system, comprising: providing anelectrical circuit, the electrical circuit comprising an alternatingcurrent (AC) voltage source having a first terminal, a variablecapacitor electrically coupled in series with the first terminal of theAC voltage source, a coil electrically coupled in series with thevariable capacitor, and a return line electrically coupling the coilwith one of a second terminal of the AC voltage source and a ground, theelectrical circuit comprising an LC circuit (resonant circuit); inresponse to the electrical circuit receiving a control signal at thevariable capacitor, adjusting a capacitance of the variable capacitor toadjust a resonant frequency of the resonant circuit between a firstresonant frequency configured to be used for wireless power transfer anda second resonant frequency configured to be used for near fieldcommunication (NFC), and; using an entire length of the coil for boththe first resonant frequency and the second resonant frequency.
 12. Themethod of claim 11, wherein adjusting the resonant frequency between thefirst resonant frequency and the second resonant frequency comprises oneof adjusting the resonant frequency from a lower value to a higher valueand adjusting the resonant frequency from the higher value to the lowervalue, wherein the lower value is between 5.800 MHz and 7.525 MHz andwherein the higher value is between 9.625 MHz and 14.850 MHz.
 13. Themethod of claim 11, wherein the variable capacitor comprises a bariumstrontium titanate (BST) variable capacitor.
 14. The method of claim 11,wherein the coil comprises an antenna.
 15. The method of claim 11,wherein the variable capacitor does not comprise a switched capacitor,does not comprise a varactor diode, and does not comprise a trimmercapacitor.
 16. The method of claim 11, wherein adjusting the resonantfrequency between the first resonant frequency and the second resonantfrequency comprises one of opening and closing a switch of theelectrical circuit.
 17. The method of claim 16, wherein one of openingand closing the switch electrically couples a fixed capacitor inparallel with the coil.
 18. The method of claim 11, further comprisingtuning the capacitance of the variable capacitor to adjust one of thefirst resonant frequency and the second resonant frequency in responseto the resonant circuit being out of resonance with a second resonantcircuit of a computing device.
 19. The method of claim 18, whereintuning the capacitance of the variable capacitor comprises one ofadjusting the first resonant frequency from a first value to a secondvalue and adjusting the second resonant frequency from a third value toa fourth value, wherein the first value and the second value are between5.800 MHz and 7.525 MHz and wherein the third value and the fourth valueare between 9.625 MHz and 14.850 MHz.
 20. The method of claim 11,wherein the electrical circuit comprises a near field communication(NFC) chip coupled with the resonant circuit and wherein adjusting theresonant frequency to the second resonant frequency protects the NFCchip from wireless power transfer signals from a power source bydetuning the resonant circuit from the wireless power transfer signals.